Licensed Shared Access (LSA) was recently developed by the European Radio Spectrum Policy Group (RSPG) to propose a new way for answering to the mobile operators' needs for more spectrum. Indeed, it is expected that no more dedicated spectrum will be available for cellular operators for mobile communications in the future. In order to address this, LSA proposes mechanisms for introducing shared spectrum based solutions, for example mobile cellular operators will have access to additional licensed spectrum from other licensees such as public safety, government, and so on, to which mobile cellular operators normally would not get access. Another approach to shared spectrum is Authorized Shared Access (ASA). ASA, however, is limited to IMT spectrum whereas LSA also addresses non-IMT bands. LSA and ASA are currently only at the conceptual level. Another related technology is called Cloud Spectrum Services (CSS) which addresses the same framework as LSA and ASA, but introduces more detailed implementation solutions. It should be noted that the approaches of LSA/ASA/CSS are merely examples of shared spectrum access.
The approaches of LSA/ASA/CSS will introduce mechanisms for enabling Mobile Devices (MDs) to share spectrum under a licensed regime. For example, following the CSS concept, the spectrum may be allocated based on various time-scales, ranging from static up to a highly dynamic allocation of shared spectrum. One issue of spectrum sharing, however, lies in the fact that a primary user, typically, the spectrum owner, the incumbent, can choose to reclaim any of its spectrum at any time. Also, the primary users might want to be able to allocate any parts of the considered spectrum to their own services. In such a context, the choice between Frequency-Division Duplexing (FDD) and Time-Division Duplexing (TDD) for Alternate Spectrum Holders (ASHs), such as Long Term Evolution (LTE) in the current 2.3-2.4 GHz band, typically depends on the behavior such as the spectrum usage characteristics of the primary user, the incumbent. In some contexts, FDD use may refer to paid operation, and TDD use may refer to unpaired operation. Paired operation may indicate that a channel or a block of spectrum in a lower frequency may be associated with a channel or a block of spectrum in a higher frequency, typically separated by a duplex distance, for example an uplink channel block and a downlink channel block, whereas unpaired may indicate no such association. Furthermore, TTD use may be implemented in either unpaired or unpaired operation. It should be noted that sometimes the primary user may temporarily grant exclusive or quasi-exclusive use of spectrum to an alternate spectrum holder during a licensed period in which the alternate spectrum holder or user may have the same rights to the spectrum as the primary spectrum holder such that the alternate user has primary or quasi-primary rights to the spectrum during such use. Current documents on LSA/ASA/CSS do not foresee any intelligent management of FDD/TDD spectrum access. Also, other dedicated spectrum based technologies do not dynamically change between an FDD/TDD type of spectrum access. Consequently, the legacy solutions rely on a fixed, pre-defined FDD or TDD spectrum access. In a non-static spectrum sharing context, such a fixed solution can either lead to a poor exploitation of the available resources or even to the worst case that the concerned spectrum resources cannot be used for a target service.
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